Physics-1998-Set-III

(Only those questions have been given, which are different from Set I)

Q2) Draw the energy band diagram for a p-type extrinsic semi-conductor. (Marks 1)
Ans2) Energy band diagram of p-type semi conductor

Q8) Two wires A and B of the same length and of the same metal have their areas of cross-section in the ratio of 1 : 4 respectively. If the same potential difference is applied across the terminal ends of each wire, what will be the ratio of the current flowing through A and B.
(Marks 1)

Ans8) RA/RB = AB/AA = 4 : 1
As current is inversely proportional to resistance
IA/IB = RB/RA = 1/4

Q14) State the laws of photoelectric emission. (Marks 2)
Ans14) Laws of photoelectric emission :
(i) For a give metal & frequency of incident radiation, the no of photoelectrons ejected per second is directly proportional to the intensity of incident light.
(ii) For a given metal there exists a certain min frequency of the incident radiation called threshold frequency below which no emission of photoelectrons takes place.
(iii) Above the threshold frequency the max kinetic energy of the emitted photoelectrons is independent of the intensity of the incident light but depends upon the frequency of the incident light.
(iv) The photoelectric emission is an instantaneous process.

Q16) How does the self-inductance of a coil change when :
(i) the number of turns in the coil is decreased ?
(ii) an iron rod is introduced into it?
Justify your answer in each case. (Marks 2)

Ans16) (i) Self inductance is directly proportional to the square of the no of turns of the coil, hence with the decrease in the no of turns of the coil, self inductance decreases.
(ii) When iron is introduced in the coil, self inductance increases as the permeability of self iron is about 1000 times

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